Lung surfactant protein A is an important structural component of mammalian lungs. The carboxy terminal domain of this protein recognizes and binds carbohydrate at neutral pH and in the presence of Ca2+. This domain also shows significant sequence similarity to the carbohydrate recognition domains of other animal lectins which require neutral pH and Ca2+ to bind carbohydrate. The precise function and mechanisms of the carbohydrate recognition domain (CRD) of lung surfactant protein A are unknown, however, evidence that this protein plays a role in establishing and maintaining the architectural integrity of the lung as well as in clearing the lung of invading pathogens does exist. The goal of this project is to define a molecular-level structural basis for these observed functions via the X-ray crystallographic structure determination of the CRD of human, rat, and mouse lung surfactant protein A.